This invention relates to the field of nondestructive testing by optical speckle interferometry. The invention provides a method and apparatus which reduces or eliminates the effect of noise on the test results, while still providing a sharp image which can be viewed in "real time" on a video display.
Optical interferometric techniques have been known for nondestructive testing (NDT) of objects. Two of these techniques include electronic speckle pattern interferometry (ESPI) and speckle shearing interferometry, also known as shearography. In ESPI, a laser beam is directed towards the test object, and the light reflected from the object is made to interfere optically with a reference beam which does not touch the object. For a diffuse, opaque object, the interference pattern typically contains speckles, and the result is therefore called a speckle pattern. Analysis of the speckles provides information about the structural integrity of the object.
In shearography, two images of the same object are taken, but the images are laterally displaced, by a small amount, relative to each other, and made to interfere. The interference pattern gives information about the state of the object. Because the two images are laterally displaced, or "sheared", the interference pattern is known as a "shearogram" and the technique is called shearography.
Both ESPI and shearography are powerful tools that can be used to detect hidden defects in aircraft parts, turbine blades, space vehicles, automobiles, and many other products. However, such techniques are extremely susceptible to noise that permeates most industrial environments. The latter problem arises because speckle interferometric techniques are comparative methods which analyze the object during two different states of stress. A finite period of time is needed to record information about the condition of the object at a given level of stress, and to induce a change in the state of the object. Any noise that occurs during this interval degrades the resulting signal. The longer the interval, the poorer the quality of the signal.
The most commonly used interferometric technique involves subtraction. Two images, each representing information about the state of the object at a particular level of stress, are subtracted from each other digitally, pixel by pixel, to produce a second-order image. In a noise-free environment, this second-order image appears on a video screen as a set of bright interference fringes having excellent contrast. However, the digitization process restricts the time interval between states to be no shorter than an interval implied by the video framing rate, which is typically 30 Hz. Therefore, any noise of higher frequency than the video framing rate will distort the signal and will significantly reduce the signal to noise ratio of the system.
An additive technique has been proposed to reduce or eliminate the problem of noise. In an additive process, the images corresponding to the object in two different states of deformation are added together, in an analog manner, i.e. by double exposure. Since there is no need to digitize the images before addition, the process can be performed at a substantially faster rate than in the subtractive technique. The additive method therefore provides excellent suppression of noise having frequencies of the order of kHz. Unfortunately, interference fringes obtained from an additive process have very poor contrast and are essentially invisible without some kind of post-process filtering. Such post-exposure processing is a difficult task, and typically yields low-quality information. For this reason, the additive technique has not been used commercially.
The present invention provides a method and apparatus which enjoys the benefits of both the subtractive and additive processes described above, and which can be practically implemented in an industrial environment. The present invention provides superior environmental noise reduction, improved signal contrast and brightness, and can be used as a full-field optical technique in the field of nondestructive testing.